Moirxc3xa9 fringes, which can be formed between a grating and its reproduction on a test piece, provide measurements of the test piece""s surface topography similar to interference fringes but at larger contour intervals.
A grating can be projected or shadow cast onto a test surface and the same or a different grating can be imaged together with the test surface for producing moirxc3xa9 fringes that follow contours of the test surface. The images of two gratings are involved. Ordinarily, one is an actual grating, which has a planar form, and the other is an image of the same or a similar grating, which takes the form of the test surface.
The actual grating contains parallel equally sized and spaced bands (referred to as lines) that alternately block or transmit light. The corresponding lines of the imaged grating depart from such regularity as a function of their relative displacement on the test surface. The difference between the planar form of the grating as a reference surface and the nominally planar form of the test surface appears as fringes formed by a superposition of the two gratings (i.e., simple addition of the overlapping grating lines in a given direction of view).
A full spectrum of visible light can be used for purposes of illumination. Temporal coherency of the light is not required unless a projected grating is itself formed by an interference pattern. Ordinarily, the grating image is formed on the test surface either as a projection of an actual grating or as a shadow pattern of an actual grating that overlies the test surface. The grating projections are preferably made by telecentric illumination systems. The shadow patterns are preferably cast by collimated light. Both approaches minimize perspective distortions and magnification errors associated with imaging the grating on test surfaces that depart from a planer form.
A camera records the grating image on the test surface together with the actual grating or a reference grating. The images of the test surface and the grating variously depart from each other in a direction of the depth of focus of the viewing system. Accordingly, the camera is preferably incorporated within a telecentric viewing system for minimizing viewing angle errors.
The grating lines are generally spaced apart at periods of approximately 50 microns or more to avoid the preponderance of diffractive effects on the light passing through the gratings. Contour intervals between fringes (i.e., the displacement represented by adjacent fringes) are a function of the grating period as well as the angles of illumination and view.
The separate systems for illuminating and viewing the grating and test surface can be costly, especially if telecentric or collimating requirements are imposed upon the systems. For example, the illuminating systems generally require either telecentric optics for projecting accurate grating images or collimating optics for casting accurate shadows from the gratings. The viewing systems also generally require telecentric optics to avoid viewing angle errors between axially displaced areas of the test surfaces.
Our invention among its various embodiments simplifies moirxc3xa9 interferometers by using a common focusing optic for both illuminating and viewing a grating pattern on a test surface. For purposes of illumination, the common focusing optic conveys light in a form that accurately reproduces the grating pattern upon the test surface. For purposes of viewing, the common focusing optic contributes to the formation of an accurate image of the grating pattern appearing on the test surface from a different perspective.
Costs normally associated with purchasing and assembling separate focusing optics for accomplishing the illuminating and viewing functions are significantly reduced by utilizing a common focusing optic for both functions. In addition, the common focusing optic renders the interferometer less sensitive to environmental influences that could otherwise affect the two functions more differently.
An exemplary moirxc3xa9 interferometer for measuring flatness of a test surface according to our invention has an illuminating system for illuminating the test surface and an imaging system for imaging the test surface onto a fringe pattern detector. The illuminating system includes a light source and an optical pathway for conveying light embodying a grating pattern to the test surface. The imaging system includes an optical pathway for relaying an image of the grating pattern on the test surface to the fringe pattern detector.
A focusing optic positioned along an intersection of the optical pathways conveys light to and from the test surface. Light conveyed from the light source through the focusing optic casts or projects the grating pattern onto the test surface. Light conveyed from the test surface through the focusing optic images the grating pattern appearing on the test surface onto the detector.
Fringes representative of the test surface""s topography are formed by a combination of the grating pattern appearing on the test surface and a reference grating pattern, which can be viewed simultaneously with the grating pattern on the test surface or simulated by computer processing. The grating pattern appearing on the test surface can be formed in a variety of ways such as from an actual grating, from a combination of actual gratings, or from an interference pattern. Once formed, the grating pattern can be projected or cast onto the test surface. The reference grating pattern can be formed from the same or a similar grating pattern.
According to one version of the invention, the focusing optic both collimates light en route to an overlying grating and test surface and telecentrically images onto the detector a moirxc3xa9 fringe pattern formed by a combination of the grating and its shadow cast upon the test surface. The collimating function provides for casting a high contrast shadow pattern of the grating upon the test surface. The telecentric imaging function increases dimensional accuracy of the imaged fringe pattern by reducing viewing angle and magnification errors associated with variations in the surface in a direction of the depth of focus.
According to another version of the invention, the focusing optic functions both as a telecentric projector and as a telecentric imager. As a telecentric projector, the focusing optic projects an image of the grating pattern onto the test surface at a well defined angle. As a telecentric imager, the focusing optic relays a dimensionally stable image of the grating""s appearance on the test surface to the fringe pattern detector. The re-imaged grating pattern taken from the test surface is superimposed on a reference grating pattern to produce the required moirxc3xa9 fringe pattern at the detector. Alternatively, the reference grating pattern could be effectively superimposed during subsequent processing.
The grating pattern cast or projected onto the test surface is preferably formed by an actual grating with equally sized and spaced lines producing a 50 percent duty cycle of transmission and non-transmission (e.g., reflection, absorption, or scattering) laid out on a planar substrate. The illuminating system reproduces the grating line pattern on the nominally planar form of the test surface by shadowing or projection. If reproduced by shadowing, the actual grating overlies the test surface and is imaged as a reference grating together with its shadow cast upon the test surface for producing the required moirxc3xa9 fringe pattern. If reproduced by projection, the actual grating is projected onto the test surface from a more remote location, and the same or a similar grating is superimposed by the imaging system as a reference grating for producing the required moirxc3xa9 fringe pattern.
The grating can also be a virtual grating formed by the mechanism of interference. For example, a beam of coherent light can be divided into two planar wavefronts that are recombined at a controlled angle to produce a pattern of linear fringes. The test surface is located within a region of overlap so that the interference fringes appear on the test surface. Another interference pattern generated by a reference surface or a reference grating can be superimposed on an image of the fringes appearing on the test surface to produce the required moirxc3xa9 fringe pattern. The focusing optic participates in both conveying the underlying light of the planar interfering wavefronts to the test surface and conveying the appearance of the interference pattern on the test surface to the detector.
Although the focusing optic can be arranged to work in either a transmissive mode or a reflective mode, the reflective mode is preferred for illuminating and imaging larger test surfaces. For example, the focusing optic can be formed as a concave spherical mirror having operating dimensions matching those of the test surface.
The optical pathway of the illuminating system intersects the focusing optic at a first angle, and the optical pathway of the imaging system intersects the focusing optic at a second angle. The two angles differ from each other to provide for separately controlling the angle at which the test surface is illuminated from the angle at which the test surface is viewed.
A directional optic located along the optical pathway of the illuminating system provides for controlling an incident angle of the light illuminating the test surface. The primary purpose of the directional optic is to redirect the light from the focusing optic on a path towards the test surface. However, the directional optic can also be arranged as a two-surface reflecting element for forming interference fringes. Another purpose of the directional optic is to provide for phase shifting. The directional optic can be pivoted to vary the incident angle of the light in increments totaling at least one fringe spacing. The detector captures images of the fringe pattern at e of the incremental angles of incidence, and the patterns are evaluated to obtain measures of surface flatness.